This invention relates to an image processing system which is for use in combination with a recording medium, such as a compact disk read-only memory (CD-ROM), or a magnetic video tape, and which comprises an image recording system and an image reproducing system for recording on and reproducing from the recording medium a sequence of recording signals.
In copending U.S. patent application Ser. No. 189,249, filed May 2, 1988, and assigned to the present assignee, Toshio Koga et al. have proposed an image processing system of the above-described type wherein a sequence of image signals is recorded on a CD-ROM by the use of an efficient coding technique and is reproduced in an offline manner. Such system will be referred to as an offline image processing system. With this system, intraframe coding is skillfully used in combination with interframe coding to code the image signal sequence into a sequence of recording signals which has a remarkably reduced amount of information. According to the offline image processing system, it is possible to reproduce a sequence of images from the CD-ROM for a long time of, for example, one hour or so. In addition, the offline image processing system enables a reverse reproduction, a high speed reproduction, a skip reproduction of the image sequence, and the like. At any rate, the CD-ROM can be used in the offline image processing system as well as a video tape.
Meanwhile, the image signal sequence usually carries a rapidly moving image and a comparatively stationary image which slowly varies in comparison with the rapidly moving image and which may be called a background image. As known in the art, such a background image may be specified by a small amount of information as compared with a rapidly moving image. This shows that a further redundancy reduction can be achieved by distinguishing between the background image and the rapidly moving image and by coding the background image in a manner different from the rapidly moving image.
However, no consideration has been made in the above-referenced United States Application as regards processing the background image which can be distinguished from the rapidly moving image.
In Hideo Kuroda et al. U.S. Pat. No. 4,591,909 issued an interframe coding method is disclosed wherein a background image is distinguished from a moving image and is selectively subjected to predictive coding. The interframe coding method might be effectively used in a television conference system wherein a sequence of coded signals is transmitted from a transmitting end to a receiving end in real time or in an on-line manner. In this connection, the above-mentioned method may be called an online coding method.
However, the online coding method might not be always optimum on recording a sequence of images and for reproducing the recorded images in an offline manner. This is because a current background image signal is determined in the online coding method at a current time instant only in dependence upon a sequence of past image signals preceding the current background signal and might be determined in an offline image processing system not only in dependence upon the past image signals but also in dependence upon a sequence of succeeding image signals.